They break down organic matter into valuable compost, they aerate the soil, they provide valuable nutrients. Worms are great! But, it turns out, that doesn’t apply to all worms. Meet the Asian jumping worm.

Most earthworms are not native to the great growing regions of the United States and Canada; settlers brought earthworms with them from other countries, and the most important worm species for farmers and gardeners are from Europe. The Asian jumping worm—the name, confusingly, refers to a number of fairly similar species of worm—originated in Japan and Korea, and probably came over to North America sometime in the 19th century. But it’s only become common in large numbers in the past decade, having first been found in Wisconsin in 2013.

The various species of Asian jumping worm are a dark grey-brown color, and not slimy like other earthworms. Their name comes from their behavior; when disturbed, they wriggle violently in a snake-like motion, sometimes even leaping into the air or detaching their tails to try to escape. They also tend to bunch up, which means that if you see one, chances are there are a lot more nearby.

https://www.youtube.com/watch?v=jrGnUFDXuyQ

Unlike other worms, Asian jumping worms can reproduce without mating, and though they die each winter, they leave behind lots of cocoons, which look like little black seeds and are commonly moved, undetected, in soil and compost. Those cocoons hatch in the warmth of spring, maybe somewhere new.

But the big reason these worms are a concern is that they don’t really burrow the way other earthworms do. Instead, they stay on the surface, often in leaf piles. They also grow more quickly, reproduce more rapidly, and consume more plant matter than other worms. The result of an Asian jumping worm infestation is the soil transforming into a field of soft dry soil pellets, sort of like ground coffee.

By attacking the surface of the soil, they make for an inhospitable environment for insects and plants alike. Soil hit by Asian jumping worms is nutrient-rich, sure, but it’s all on the surface, and is easily washed away by rain, almost like fertilizer. It’s not a sustainable way to keep the soil rich and healthy. And the rapidly vanishing leaf cover makes an environment that’s bad for slow-growing plants like trees, and good for weeds. Researchers find that they’re also out-competing the European earthworms in some way—they’re not quite sure how, yet—and that’s bad for the soil, too.

Here’s the worst part: there’s no solution at all. Recommendations are pretty much solely for prevention, and some of the instructions are hard to follow, like diligently cleaning any soil from boots or cars, or inspecting any new soil to make sure it’s free of cocoons (which, again, are tiny and basically just look like soil). Research is moving forward to find a solution—but one isn’t here yet.